In drafting mechanisms, top arms are employed that extend over the drafting field from a rear mounting and have devices for holding and weighting consecutive dual boss top-rollers from centre necks of the rollers. For any particular fibre each such device should provide a fixed weighting pressure on the dual boss top-roller held thereby, but a different fibre may require a different weighting pressure, particularly where man-made fibres are concerned. The change in weighting required can be high, and desirably it should be possible to double the weighting as between "light weight" and "heavy weight". To obtain such a magnitude of change in devices employing springs to exert the weighting pressure, high ratio springs (i.e. springs which give a big change in pressure for a small change in compression) have been used in conjunction with mechanism operable manually to alter the compression of the springs. However, high ratio springs have the drawback that there is a tendency for the weighting pressure to change during the normal operating up-and-down movements of the top-rollers.